chore(website): migrate webpack configs to ESM

[yangshun]

Context

The website webpack config stack was still using CommonJS while the rest of the website codebase had already largely moved to lodash-es.

This updates the webpack-related config files to ESM and removes the remaining direct lodash usage from the website package.

Implementation

• rename the webpack config files and shared parts from .js to .mjs
• convert the webpack config stack to ESM imports / exports
• update website scripts and ESLint config to consume the ESM webpack config files correctly
• add a small CommonJS-only webpack resolver config for eslint-import-resolver-webpack
• migrate the remaining website lodash usage to lodash-es
• remove direct lodash and @types/lodash from website/package.json
• refresh pnpm-lock.yaml

Validation:

• pnpm exec eslint --ext .js,.mjs .eslintrc.js scripts/build.js scripts/apply-venue-updates.js scripts/fix-venue-name.js scripts/sitemap.js webpack
• NODE_ENV=development pnpm exec webpack configtest webpack/webpack.config.dev.mjs
• NODE_ENV=production pnpm exec webpack configtest webpack/webpack.config.browser-warning.mjs
• NODE_ENV=development pnpm exec webpack configtest webpack/webpack.config.timetable-only.mjs
• direct Node import of the updated webpack dev / prod configs

Other Information

I did not run a full pnpm --dir website build in this branch.

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2 Skipped Deployments

Project	Deployment	Actions	Updated (UTC)
nusmods-export	Ignored		Mar 8, 2026 8:39am
nusmods-website	Ignored		Mar 8, 2026 8:39am

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 56.30%. Comparing base (988c6fd) to head (5b60d4a).
⚠️ Report is 217 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4351      +/-   ##
==========================================
+ Coverage   54.52%   56.30%   +1.77%     
==========================================
  Files         274      308      +34     
  Lines        6076     6962     +886     
  Branches     1455     1682     +227     
==========================================
+ Hits         3313     3920     +607     
- Misses       2763     3042     +279     

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[greptile-apps]

Confidence Score: 4/5

• Safe to merge; this is a well-structured syntax-level migration with no functional regressions.
• All functional logic is preserved. The only observable runtime change is that appVersion() is now called once at module load instead of twice, which is an improvement. The dynamic lodash-es import pattern is safe given the current call order in every script. The author explicitly notes a full production build was not run; given the scope of file renames and require → import churn, a CI build confirmation would add confidence, but the changes are structurally sound.
• No files require special attention; this is a straightforward ESM migration with correct handling of CJS interop.

Sequence Diagram

sequenceDiagram
    participant CLI as Node CLI (scripts/build.js)
    participant LWM as loadWebpackModules()
    participant PROD as webpack.config.prod.mjs
    participant BO as webpack.config.browser-warning.mjs
    participant TO as webpack.config.timetable-only.mjs
    participant PARTS as webpack.parts.mjs
    participant LES as lodash-es

    CLI->>LWM: await loadWebpackModules()
    par Dynamic ESM imports (Promise.all)
        LWM->>PROD: import(...)
        LWM->>BO: import(...)
        LWM->>TO: import(...)
        LWM->>PARTS: import(...)
        LWM->>LES: import('lodash-es') → castArray
    end
    LWM-->>CLI: { production, timetableOnly, browserWarning, parts }
    CLI->>CLI: buildAll(previousDistFileSizes, webpackModules)
    par Parallel builds
        CLI->>CLI: buildProd({ production, browserWarning, parts })
        CLI->>CLI: buildTimetableOnly({ timetableOnly })
    end
    CLI->>CLI: writeCommitHash(parts)

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_{Last reviewed commit: 5b60d4a}

[ravern]

We had to revert the lodash -> lodash-es changes from #4348 (reverted in #4353) due to a runtime error in the export service.

Will hold off on this PR for now while investigating.

[yangshun]

Yes, thanks for the reminder. I'll also be investigating.

[ravern]

Opened a PR here: #4357

[yangshun]

Let's not bother with this since the noisy warnings no longer show up during development and build (cuz the Babel plugin has been removed).